1. Field of the Invention
The present invention relates to an image printing apparatus and image printing method of an inkjet printer that prints an image by discharging ink onto a printing medium, and more particularly to control of glossiness.
2. Description of the Related Art
Printing an image by an inkjet printer is performed by a printing head discharging small drops of ink onto a printing medium. A method of using piezoelectric elements that generate mechanical strain through applying a voltage, and a method of rapidly heating and vaporizing ink and then using high-pressure air bubbles that are generated are widely used as methods for discharging small drops of ink from a printing head.
The ink that is used in an inkjet printer often uses dye that is easily dissolved in water, however, in recent years, with the aim of improving the light, gas and water resistance of images, pigments are also often used. In pigment type ink, it is known that a coloring material does not easily penetrate into the printing medium and remains on the surface of the printing medium. Therefore, in an image that was printed using pigment ink, the surface shape is different between the non-printed areas and printed areas, and even between the printed areas where a lot of coloring material is used and where less coloring material is used, as a result, uneven glossiness occurs.
The intensity of the glossiness changes due to the refractive index of the material and the surface shape, such that the higher the refractive index is the more intense the glossiness becomes, and the smoother the surface shape is, the more intense the glossiness is. Moreover, in regards to the smoothness of the surface shape, depending on the blurriness of a photographed image, the glossiness felt will change, so the blurriness of a specular reflection image is expressed by an index called gloss clarity that indicates a degree of glossiness. Generally, a person senses glossiness from both the intensity of specular reflected light (corresponds to specular glossiness that is given by the specular glossiness measurement method (JIS Z 8741) and the gloss clarity. In other words, glossiness is expressed by both specular glossiness, which is affected by the refractive index of the surface and also the surface shape, and the gloss clarity (blurriness) that changes according to the surface shape.
Conventionally, a method of installing colorless and transparent ink (hereafter referred to as colorless ink) that does not include any coloring material in a printer has been proposed as a method for eliminating uneven glossiness. In this method, this colorless ink is discharged onto an image together with ink that includes coloring material (hereafter referred to as colored ink) such as CMYK ink. By using colorless ink, glossiness of the image is controlled by controlling the smoothness of the surface of the image without affecting the color of the image, and controlling the refractive index of ink that exists on the outermost surface of the image that greatly affects the specular reflection. The mechanism for this is illustrated in FIGS. 1A to 1D. In FIGS. 1A to 1D, FIG. 1A is a cross-sectional diagram of the printing medium when an image is formed on the printing medium (having a refractive index of n2) using only colored ink (indicated in black and having a refractive index of n1), and FIG. 1B is a cross-sectional diagram of the printing medium when an image is formed also using colorless ink (indicated in white and having a refractive index of n3). The colorless ink does not affect the color, so there is no difference in the color of these two images. As illustrated in these drawings, when an image is formed using only colored ink, and when color is reproduced in half tones so that the ink dots exist in a sparse manner, a difference occurs in the height of the surface of the printing medium and the surface of the ink due to the characteristic of pigment coloring material accumulating up on the surface. Diffused reflection of light occurs on the surface due to the unevenness in height, and as a result the glossiness, and particularly the gloss clarity, worsens. On the other hand, in the image in FIG. 1B where colorless ink is also used, the printing medium is covered by colorless ink, so when compared with the image in FIG. 1A, unevenness of the surface is reduced and gloss clarity is improved. Furthermore, generally the refractive index becomes higher in the order printing medium (n2)<colorless ink (n3)<colored ink (n1). Therefore, when compared with the difference in the refractive index (n1−n2) in the image in FIG. 1A, the difference in the refractive index (n1−n3) in the image in FIG. 1B is lower, and as a result, the unevenness in the glossiness, and particularly the specular glossiness, is suppressed, and often the overall specular glossiness becomes high.
In the case of pigment ink, the amount of pigment particles accumulated on the surface of the paper and the refractive index of the ink differ, so generally the glossiness is different for each ink. FIG. 1C is a cross-sectional diagram illustrating an example of a printing medium when a plurality of inks having different characteristics exist on the printing medium. The dots in the cross-sectional diagram illustrated in FIG. 1C not only have different indices of refraction such as n1, n4 and n5, but also the heights and shapes when accumulated differ due to the amount of solids such as coloring materials and polymers in the pigment inks. The difference in glossiness in that case is explained with reference to FIG. 2. In FIG. 2, the horizontal axis is the amount of ink that is discharged onto the surface of the paper, and the vertical axis is the specular glossiness. The solid line 201 illustrates the transition in the specular glossiness of the primary color gradation of ink, and the dashed line 202 and dotted line 203 illustrate the transitions of specular glossiness of inks having lower indices of refraction. In this way, it can be seen that the lower the refractive index of the ink is, the lower the specular glossiness becomes.
However, even in the case of controlling glossiness using colorless ink, when the glossiness (the uneven shape and refractive index of overlapping ink) is not uniform for each ink as illustrated in FIG. 1C, it is difficult to increase the uniformity of glossiness with only one kind of colorless ink. In other words, even when colorless ink is arranged between dots of ink as illustrated by the cross-sectional diagram of printing medium in FIG. 1D, in which a colorless ink is further applied to the printing medium, the differences in the indices of refraction become sparse as n1−n3, n4−n3 and n5−n3. As a result, the heights of the dots of colored ink also become non-uniform. On the other hand, using a number of colorless inks equal to the number of ink colors in correspondence to the indices of refraction of the inks and the heights of the dots is not a realistic method.
Therefore, the object of the present invention is to improve the uniformity of glossiness of a printed surface by controlling printing in consideration of the differences in the indices of refraction of the inks and the unevenness of the surface.